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Embryology
Making and growing babies
Embryology
The study of formation, growth, and development of organisms
• Variety of strategies among species
• Reproduction can be asexual or sexual
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Asexual Reproduction
Only the genes of one parent are passed on
Offspring are clones of that parent
Types:
• Binary Fission
• Budding
• Fragmentation/Regeneration
Binary Fission
Bacteria double their DNA and then split in two, with one complete set of DNA in each cell
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Budding
Offspring grows from the body of a parent
• Sponges
• Hydra
Fragmentation/Regeneration
Parent breaks into pieces that each become a new organism
• Planarians
• Sea Cucumbers/sea stars
• Sponges
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Sexual Reproduction
Two individuals produce offspring that have genetic traits of both parents.
New combinations are produced
Mammals, fish, amphibians, birds, reptiles
Sexual Reproduction begins with the fertilization of the egg by the sperm
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Fertilization
The merging of the sperm with the egg
Can be external (outside of the female)
Can be internal (inside the female)
Fertilization produces a zygote that has genes and traits of both parents
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External Fertilization
Sea Urchins release sperm and eggs into the water. The sperm find and enter the eggs.
Female salmon lay eggs in a ‘redd’ while male salmon deposit their sperm on top of the eggs.
Both die afterwards.
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Internal Fertilization
Requires copulation (in mammals) or cloacal fertilization (in birds, reptiles, and amphibians)
Fertilization can result in a zygote that becomes an embryo
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Fertilization
An egg and a sperm are both haploid cells
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When the sperm fertilizes the egg, the two merge and form a diploid cell called a zygote
Diploid zygote has 2 sets of chromosomes – one set from each parent
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Sperm
A nucleated head with DNA and a tail (flagella)
Flagella moves the sperm through liquid to the egg
Mitochondria provides energy for movement
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Sperm and Semen
Sperm created in testicles
Human males produce over 100 million sperm daily
Semen = sperm + fluids. Semen protects sperm & provides energy and mobility
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Eggs
Females produce all their eggs before birth (1-2 million)
Eggs are partially developed
After puberty begins, 1 egg per month completes development and is released
Ovulation occurs ~2 weeks after menstruation begins
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Copulation
Sperm is transported in semen into vagina
Sperm travels though uterus to fallopian tubes where fertilization occurs
Of over 100 million sperm, only a few dozen will reach the egg
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Fertilization
Sperm bind to outer coating of egg
Acrosome eats through egg membrane until one gets through and enters the egg
Fertilization
Egg has instant chemical change and becomes impermeable to remaining sperm
Sperm and egg nuclei unite, forming zygote
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Development
Initial development in fallopian tubes
Implantation in the uterus occurs after ~ a week
Zygote divides through mitosis in a process called cleavage.
Once the zygote divides, it is known as an embryo.
The cells continue to divide until a ball of cells forms. This is called a blastula.
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Gastrula
Blastula folds inward, forming a gastrula
Similar to a potter pushing in on a lump of clay to form a cup.
Endoderm = the interior of the cup
Ectoderm = outside of the cup
Mesoderm = formed from the middle
The opening becomes the mouth
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Advanced animals:
Gastrula develops a middle layer (mesoderm)
Mesoderm becomes muscles, and body systems (circulatory, excretory, and respiratory).
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Induction/Differentiation
Cells differentiate into specific types of cells according to genetic instruction
Heart, lung, liver cells can develop from any cell in the body
Induction is the process of all the cells in the body being told what to become
History of Development
Spemann & Mangold
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Tied baby hair around new newt embryo to sever communication between halves
Each half developed into complete newt
Experiment showed that any cell in the embryo can develop into a complete individual
Spemann received a Nobel prize
Mangold died in a fire at 26 – Nobel prize cannot be awarded post humorously
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Embryo Development in Twins
Fraternal twins: two or more eggs released and fertilized by sperm. Not genetically identical
Identical twins: one egg fertilized but split during early mitosis. Identical genetics
Siamese (congenital) twins: egg splits late in development, and separation is incomplete
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Growth of the developing fetus in humans
As the embryo grows, it must be nourished and protected
The exchange of blood and nutrients between mother and baby occurs through the placenta
Growth and Development
First Trimester
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Week 3: Implantation
Week 5: Fetus heart beats, brain forming, arm/leg buds visible
Week 7-8: major organs, embryo can move; the fetus weighs less than an aspirin
Week 9-12: fingers/toes form and fetus can make a fist
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Second Trimester
Week 13-16: fetus can suck, swallow, and feel pain. Fetus does kicks/somersaults
Week 20: Fetus can hear and recognize mom’s voice
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Third Trimester
Week 24: Fetus has fingerprints
Week 25: Fetus opens and closes eyes
Week 26: Fetus can possibly survive on its own if necessary. The fetus weighs about 2 1/2
pounds
Week 38-40: full term
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Inheritance of traits
When the egg and sperm unite, the zygote receives one set of DNA from both parents
DNA is the set of instructions that dictate physical traits
DNA is divided into chromosomes
Chromosomes are divided into genes
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Inheritance
Humans have 46 chromosomes arranged in 23 pairs
One chromosome in each pair comes from the mom, and one comes from the dad
The way the chromosomes combine determines the physical traits
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Genes and Alleles
Genes are on chromosomes
Each person has two copies of each gene
Each gene codes for one protein/trait
Genes may have different variations for that particular trait
Example: there is a gene for cleft chin
Variations of the cleft chin gene: you have it; you don’t have it
These variations of a gene are called alleles
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Examples of Genes and Alleles
Gene
Alleles
Eye Color
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Blue, Green, Hazel, Brown
Hand Preference
Left, Right, Both
Widow’s Peak
Present, Absent
Blood Cell Shape
Round, sickle-shaped
Dominance
Genes (alleles) can be dominant or recessive
Dominant alleles are always expressed if present in the individual
Dominant alleles are represented by uppercase letters
Recessive alleles are only expressed if the dominant allele (gene) is absent
Recessive alleles are represented by lower case letters (the same letter as the dominant allele)
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Traits
To have a recessive trait, the individual must have two recessive alleles – one from each
parent
If the individual has at least one dominant allele, he will have the dominant trait
For example:
• D/D = has the dominant trait
• d/d = has the recessive trait
• D/d = (carrier) has the dominant trait but can pass the recessive gene onto an offspring
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Traits and Parents
A parent with a recessive trait cannot pass on the dominant allele.
A parent with the dominant trait may pass on a recessive allele if he/she is a carrier
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Dominant/recessive disorders
Sometimes disorders are dominant. If one parent passes on the dominant allele, the child will
have the trait
Recessive disorders can only be inherited if both parents pass the allele to the child
Genetic testing and counseling helps couples decide if there are unacceptable risks in having
offspring
Inbreeding
Both parents have the same genetic background
Likelihood of both parents having the same rare, destructive recessive allele is increased
Parents are more likely to pass on recessive disorders because of similar genes
Examples of Genetic Disorders
•Dominant Disorders/Traits
• Marfan Syndrome
• Disorder of the connective tissues
• Cleft Chin
• If both parents carry trait, child has 75% chance of also having it
•Recessive Disorders/Traits
• Cystic Fibrosis
• Disease that interfers with breathing & causes early death
• Albinism
• Lack of melanin pigment in the skin, hair, nails
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Chromosomal Abnormalities
Abnormal numbers of chromosomes results in defects or death
• Mistakes usually occur during meiosis
• Can involve one chromosome pair, or entire sets of chromosomes
Extra or Missing Chromosomes
• Chromosomal abnormalities usually result in spontaneous abortion
• Monosomy – one missing chromosome in a pair, resulting in in 45 total chromosomes
• Trisomy – one extra chromosome in a pair, resulting in a total of 47 chromosomes
Other Chromosome disorders
Klinefelter Syndrome
• male born with one or more extra X chromosomes in the 23rd pair
Turner Syndrome
• Missing an ‘X’ in the sex chromosomes
• girls are ‘X’ instead of ‘XX’(monosomy)
Patau Syndrome
• Trisomy 13 – lots of facial and nervous system abnormalities. Most don’t survive gestation,
and few survive first year
Down Syndrome
• Trisomy 21: flat facial features, upward slant to eyes; enlarged tongue, heart defects; hearing
or eyesight problems; mental retardation; poor muscle tone; loose joints
Extra or Missing Sets of chromosomes (the whole set, not just a single chromosome)
• Extra or missing sets of chromosomes result in death for humans
• Monoploidy – missing the entire set from either mom or dad (23 total chromosomes)
• Polyploidy – extra sets of chromosomes
• Triploidy – one extra set from mom or from dad (69 total chromosomes)
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Other Chromosomal Problems
Translocation: part of one chromosome breaks off and attaches to a different pair
Inversion: part of a chromosome arm breaks off and reattaches in inverted position
Deletion: part of a chromosome is missing
Duplication: part of a chromosome is duplicated
Causes for Abnormalities
Error in cell division:
• Mitosis in the body cells. One dividing cell results in two identical offspring cells
• Meiosis (produces gametes) that divides the number of chromosomes in half.
Maternal Age: Because eggs are produced before birth, genetic problems can occur as they
age. Genetic problems are most common in children born to older women
Environment: a possibility that the environment may play a role in the occurrence of genetic
errors.
Diagnosing Abnormalities
Amniocentesis: 14th week of pregnancy or later. Amniotic fluid is taken from around fetus and
grown in a petri dish. Abnormalities are noted
Karyotype
• The karyotype can help doctors and geneticists identify genetic diseases and deformities
• Missing, extra, or damaged chromosomes create physical or mental problems, or even death
• Try to identify the chromosomal abnormality in the following karyotype:
• A karyotype shows the pairing of chromosomes for one person
What a karyotype would show in a patient with Trisomy-21:
• Three chromosomes in the 21st pair, and 47 total chromsomes
• Also called Down Syndrome
• Leads to physical defects and mental retardation
Ethics in Reproduction and Embryology
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In Vitro fertilization
Cloning
Stem Cell Research
Surrogate pregnancy
Abortion
Chimera Ethics
In vitro fertilization
Allows infertile couples to have a baby that is biologically and genetically related
Louise Brown = first test tube baby born in 1981 in Great Britain
Since then, more than 250,000 in vitro babies born
Technique for In Vitro Technique
• Eggs are aspirated from ovary
• Man produces semen in a cup
• Sperm and Egg are placed in a lab dish for several days
• After sperm fertilizes egg, the embryo is implanted in woman’s uterus
• Usually 2-4 embryos are implanted
• Embryo is allowed to develop naturally
In Vitro Fertilization facts:
• Cost = ~$12,500 per implantation
• Risk to child - insignificant – most babies are healthy and normal
• Risk to mom – almost none
• Other factors – incident of twins is high, and triplets are higher than with normal pregnancy
Ethical Issues of In Vitro Fertilization:
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Embryos that are not implanted are destroyed
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Slight increased risk for birth defects
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Social implications
Surrogacy
Definition: agreement where a woman gestates and gives birth to a child for others to raise
Two types:
• the surrogate contributes the egg and is inseminated with the sperm of the intended father
• The embryo is created from the sperm and egg of the intended parents through IVF and is
implanted in the surrogate
Surrogacy facts
• Cost is normally ~$10,000 – $20,000
• Additional costs for IVF, medical care, and fertility costs may reach $60,000
• Surrogates are often relatives of the parents-to-be
Ethical Considerations of surrogacy
• What if the surrogate refuses to give up the child?
• What happens if the parents change their minds?
• What happens in the case of multiple births?
• What if the child has severe defects?
• What about the rights of the child?
• Is surrogacy the same as buying a child?
• What are the ethics of artificial insemination?
Stem Cell Research
Stem cells are cells that can differentiate into a wide range of cell types
Stem cells can have the following origins:
• Embryonic
• Umbilical
• Adult
• Uses for Stem Cells
• Stem cells can be used to regenerate damaged tissue that does not regenerate naturally
• Heart (after heart damage)
• Nervous system (spinal or head injuries)
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Cloning
Creating an animal that is genetically identical to the parent
First animal (tadpole) cloned in 1952
First recognized as a possible human procedure in 1996 with the creation of “Dolly the sheep”
Success rate is very low (Dolly was the only success of 276 tries)
Cloning technique
• DNA is removed from parent cell
• The DNA is placed inside a second cell that has been “gutted” (nucleus removed)
• Chemicals or electricity are used to encourage cell division, simulating cleavage
• The embryo is transferred to a uterus (usually in a surrogate) for implantation after it has
~100 cells
• Therapeutic Cloning Technique
• The nucleus is removed from one cell
• The DNA from a second cell, say a skin cell, is inserted into the gutted cell
• Electricity/chemicals used to coax division
• Stem cells are removed and the embryo is destroyed
• The stem cells are manipulated to become other types of cells – whatever is needed
• Benefits of Cloning
• Cloning can be used to repopulate endangered animals
• Valuable animals (dairy cattle) can be cloned
• Cloning may give another option for infertility
• Cloning can produce organs or tissues for transplant
• Cloning Risks/Ethical Considerations
• Cloning only produces viable offspring 90% of time
• Cloning is costly and inefficient
• Clones have more health problems, immune problems, are abnormally large, or have
increased tumors
• The clone’s parent is actually its twin
• Some do not believe that clones can possess a soul
• Many complications with social acceptance
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Induced Abortion
Definition: Expelling/removing a developing fetus from woman’s uterus
Approximately 1.3 million yearly in U.S.
Normally done before week 12
Fairly safe for mother when done in proper setting
Types of Abortions
• Medical: up to 49 days from conception. Done with pills that block necessary hormones and
then induce contractions
• Menstrual Aspiration: fetus is removed with syringe within first 3 weeks
• Suction Curretage: fetus is suctioned out of uterus with vacuum within 1st 3 months
• Labor-inducing abortions – late abortions done by inducing early labor in hospital
• Risks of abortion
• Low health risk to mother, especially in early abortions
• Possible complications include:
• Infection
• Incomplete abortions
• Hemorrhage
• Damage to the uterus
• Death
• Roe vs. Wade
• Jane Roe (pseudonym), a single pregnant women from Texas, challenged law that abortion
was only legal to save mother’s life
• Landmark judicial decision about privacy and abortion in 1973 overturned law
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• Decision determined that a woman can choose to have an abortion for any reason up until
the fetus is viable to live outside of the body without maternal support
• Interesting note: Roe actually had the baby and gave it up for adoption
• Advantages/benefits of abortion
• Some believe that abortion is justified when:
• The pregnancy endangers the health of the mother
• The fetus has severe deformity or birth defects
• Pregnancy is the result of rape
• Women should not be forced to carry a baby that they don’t want
• Ethical Concerns of abortion
• Some believe that abortion is unethical because:
• It goes against religious beliefs
• There are better alternatives (adoption)
• Right to life – fetal rights should outweigh the right to choose
• Because the woman made the choice to have sex, she should bear the consequences
• Abortion is murder
• Abortion is sometimes used as a method of birth control
• Some women get multiple abortions rather than changing their behavior or taking
necessary precautions to prevent pregnancy
• Pro-choice arguments
• Women’s rights – the woman has the right to control her body
• The fetus is not a person until after birth
• Unwanted children are better off aborted than neglected or abused
• Men don’t have to carry children, so women should not be forced to either
• If abortion was outlawed, women would return to back-alley abortions
Late-term Abortions
Normally defined as abortions after 20 weeks gestation
Fetus is often viable
Fetus sometimes must be killed after the abortion is complete
Three methods:
• Dialation and evacuation
• Induced labor
• Intact dilation and extraction (partial birth abortion)
• Most Common Reasons Women Give for late-term Abortions
• A deteriorating financial situation
• A change in relationship with the father
• Unawareness of the pregnancy until its later stages
• Persuasion of others, like the parents of a minor
• Inability to have an abortion earlier in the pregnancy (possibly due to a lack of funds, lack of
transportation, or a legal restriction)
• Discovery of a fetal abnormality
• Risk to the mother's life or health
• Partial-birth abortion are very controversial
• Preferred by some because it is least traumatic for mother
• Bush/congress banned these in 2003; physicians who perform them were threatened with
fines and jail time
• Federal judge declared the ban unconstitutional, so it has not been enforced
• Partial-Birth Abortion procedure
• Uterus is dilated over several days
• Physician rotates fetus to breech (foot first) position
• Body is delivered
• Before head exits, skull is punctured, suctioned, and collapsed
• Head is delivered
• Because fetus is killed prior to complete delivery, the abortion is legal
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•Other Considerations of abortion
• What about the rights of the father?
Types of Birth in Animals
The embryo can be expelled from the parent in one of three ways
• Ovipary – lay eggs that hatch outside the parent
• Vivipary – bear live young without eggs
• Ovipary – lay eggs that hatch as they are born, which resembles a live birth
Three kinds of Insect Development
Gradual Development
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Eggs hatch with babies looking just like adults.
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Molt as they grow.
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Some of the wingless insects (springtail/silverfish):
Incomplete Metamorphosis
•Three stages:
• Egg
• Nymph:
• looks similar to adult but may lack appendages/can’t reproduce
• with each molt, looks more like adult form
• Adult
• Insect examples
• Cochroaches
• Grasshoppers
• Cinch bugs
Complete Metamorphosis
•Four stages:
• Egg
• Larvae: free-living, worm-like stage (catepillar)
• Pupae: tissues/organs break down. Replaced with adult tissues. No movement/feeding.
May occur in cocoon
• Adult
• Insect examples
– Moths
– Butterflies
Amphibian Development - Metamorphosis
Internal or external fertilization
Frogs lay eggs that produce tadpoles
Development occurs inside & outside the egg
Eggs are divided into parts:
• Animal pole – the developing frog
• Vegetal pole – food supply for growing frog
• Crescent – divides the poles
•Amphibian Metamorphosis: Amphibians develop from eggs to larvae to adults
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Eggs
The Primitive Eggs
• Problems
• Primitive eggs require a liquid medium throughout development to prevent drying out
• Primitive eggs have little protection from the environment
The Amniotic Egg
•Advantages
• Helped free reptiles and birds from reliance on aquatic environment
• Provides nourishment for embryo
• Has membranes to protect embryo
• Amniotic Egg Structures
• Structures that Surround
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Amnion: membrane filled with fluid that surrounds embryo
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Shell – leathery membrane which encases and protects
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Chorion: membrane around yolk, allantois, amnion and embryo. Allows gas
exchange
• Storage Structures
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Yolk: food supply for embryo.
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Albumen: clear part of egg – additional food and water for embryo
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Allantois: holds wastes of fetus.